Now that's some determination you got there ! Turning a piece of burned Chinese turd into a working device again is not something everyone can (or would like to) do .
I see you added a socket for the UC3842... in case it burns again?

You still need to clean up the carbon deposits, though, because they can be conductive.

I fixed it!
These are the photos of the repair, not so beautiful, but it's working.
Very nice sound, wish I could keep it...

Well done! The Alesis is indeed a lovely speaker especially if you use it as nearfield (as it is intended). I have mine for 5 years now, and today one of them simply didn't switch on anymore

So I rang my friend Chris who already helped me fix a M-Audio Delta 1010 unit by replacing some fried capacitors, and he is hopefully going to have a look at it this afternoon. I also rang the support guys in UK they are quoting a very reasonable repair rate of 15 pounds / half hour.

If the Power supply is fried the whole unit would be 60 Pounds, and 56 pounds for the amp, but if they repair them in house its about half of that (trade prices). On top of that I would have to pay for it shipping from Ireland to England and back here, so I am going down the self repair route to start with.

Luigi,
You did well, have you got any ideas for my board? My 470 ohm resistor was good but R15 (.22ohms!) exploded so I replaced that using a wire wound res, is that okay? I also replaced 4 x 1n4007 diodes, 2x irf840, 1 x 330uF, 1 x 1uF, 2 x 390uF and the UC3842 ic and most of the smaller transistors. Last test it came on but only for about 20 seconds before blowing all the 1n4007 diodes again and the fuse. The heatsink was hot. Before powering on, I compared resistances with the other good board and couldn't find any anomalies. Please tell me what I'm missing and where should I be looking.

I eventually gave up chasing around the board trying to find the problem. Switch mode power supplies are beyond my understanding so I just made a conventional transformer/rectifier/capacitor supply and squeezed it onto the chassis without too much trouble. Works fine now.

I just (finally) received my 2nd pair of M1 Mk.2's last week - these ones wouldn't switch on.

Sure enough, "first generation" power supply, with the start-up cap right up against the dropper resistors coming from the +320v rail. Upon desoldering, just for kicks, i measured it. Said 220u/35v on the sleeve; the cap range on my multimeter said 69uf :roll:

I decided i'd go "all out", and used a Panasonic FR 560u/50v as a replacement 2.6A ripple / 10k hours, vs. 640mA / 2k hours on the original

I chose a more "elegant" route for the actual replacing though: desoldered the 10nF ceramic cap next to the UC3844, used its holes for the Panny, and slipped the ceramic back in underneith the board, straight on the Vcc/Gnd pins of the chip That's the area they re-located the start-up cap on the later revisions anyway (on my first pair).

For what it's worth, the coating on the current-sense resistor was starting to "crack", but no flaking or disintegration going on (just yet), "only" some board discoloration right underneith it. I added some fresh solder to its pins/pads, looked a bit "dried out" to me...

Either way, stay tuned for pics tomorrow, when i "do up" the second one (it's 3:35am here, so... )

DAm , mine fryed a few days ago , i turned it on and i just head a dam powww , no light no nothing . since it passed 2 weeks frmo warranty ..... i opened it , since i work with electronics ,i open the unit and found the fuse dead , i changed it , it exploded again , i cheked , termistor was damaged and also the fuse , just in case i looked for the diodes 1n4007 and 2 of them were in short , so i changed also the 4 diodes just in case , when i turned on the unit .. POWWWWWW ... broken fuse ,broken termistor again , also R10 =1k c10=1000pf q3=2n4401 and r18_100ohm , does are the ones i've found burned .. so now , ohh just noticed also now U1=C MODULATOR UC3844N also broken ...so my question is , were should i start ...rails of 250 volts is quiet different than rails of 19 or 5 or 3.3 volts.. so i would like to have some advises by the experts here .
this is ripping me off a lot since im not making music at all ...only one speaker and the sub doesn't help much ...
i apreciate all the feedbacks

There's a lesson in there - don't replace parts before you've found the cause of their demise

If "even" the 3844 blew, there's a pretty good chance it took one or both of the FETs with it.

I'm attaching the service manual; the PSU schematic is on page 14.

In these circumstances, in addition to having replacements ready for the obviously-blown parts, i might go as far as to pull all the parts from the primary and test them out-of-circuit, to make sure they're still ok, and that the measured values are (still) a close-enough match for the markings on them.

Hey guys, I'm a newbie to the site. Looking for guidance with my Mk2. The power supply got cooked pretty good. I've had to replace quite a few parts. I'm to the point where I'm getting power to the T1 transformer. But, not beyond that. So, I need to replace that component. Does anyone know where I can source that part? I can't seem to find the appropriate replacement based on the schematic or BOM numbers.
Thanks in advance!

Let me put it this way: silicon devices are FAR more prone to damage / "death" than transformers. And if the T1 was indeed at fault, there's a pretty good chance the damage would've been visible.

Given that, is the UC384x working? What's the Vcc reading? And also, which revision of the power supply is in your pair? If it's the first version, the C8 startup cap is between some power resistors and the FET heatsink, so it's quite likely "cooked" - replacing that (and relocating that, as per one of my earlier posts) is a mandatory first step.

You say the MOSFETs were damaged and replaced. Are the gate-driver transistors (and associated passives) still ok?

Let me put it this way: silicon devices are FAR more prone to damage / "death" than transformers. And if the T1 was indeed at fault, there's a pretty good chance the damage would've been visible.

Given that, is the UC384x working? What's the Vcc reading? And also, which revision of the power supply is in your pair? If it's the first version, the C8 startup cap is between some power resistors and the FET heatsink, so it's quite likely "cooked" - replacing that (and relocating that, as per one of my earlier posts) is a mandatory first step.

You say the MOSFETs were damaged and replaced. Are the gate-driver transistors (and associated passives) still ok?

Hi Khron666,

Thanks for the reply. Yes, I saw your original post. My C8 is right next to the power resistors and the Q1 FET/heatsink. I did replace this component already per your earlier post. But, there were other parts that were visibly damaged when I began to investigate.

I've repaired hundreds of SMPS supplies, and had only ONE bad/melted transformer so far. I support the idea that it's something else. Likely the controller. I've had several DOA 384xs when bought from dodgy suppliers.

__________________

Quote:

Originally Posted by PeteS in CA

Remember that by the time consequences of a short-sighted decision are experienced, the idiot who made the bad decision may have already been promoted or moved on to a better job at another company.

Thanks for the reply. Yes, I saw your original post. My C8 is right next to the power resistors and the Q1 FET/heatsink. I did replace this component already per your earlier post. But, there were other parts that were visibly damaged when I began to investigate.

In this power supply, the "start-up cap" is the fatter one that's in-between some power resistors and the FET heatsink. As you can read in one of my earlier posts, i chose to relocate it from there, to right next to the chip.

According to UC3844 data sheet, VCC needs to be at least 16V (plus/minus 1.5V) in order for the IC to start working and 10V (again, plus/minus 1.5V) for the IC to continue to work.

What you have sounds like a classic start-up cap issue. I suggest you replace the small electrolytic capacitor near the UC3844 IC.

Thanks, momaka.

I've already replaced the startup cap (C8). But, still no luck. When I power up the supply, the voltage across C8 is 15V, so this is within what is necessary for the 3844 to get working, according to your post.